1. Field of the Invention
The present invention relates to a system and method for electronic valve actuation (EVA) using an electromagnetic actuator having a permanent magnet, particularly for actuation of intake and/or exhaust valves of an internal combustion engine.
2. Background Art
Conventional internal combustion engines use a camshaft to mechanically actuate the intake and exhaust valves of the cylinders or combustion chambers. The fixed valve timing of this arrangement, or limited timing adjustment available for variable cam timing systems limits control flexibility. Electronic valve actuation (EVA) offers greater control authority and can significantly improve engine performance and fuel economy under various operating conditions. Electromagnetic actuators are often used in EVA systems to electrically or electronically open and close the intake and/or exhaust valves.
Electromagnetic actuators may use electromagnets or solenoids to attract an armature attached to the valve stem. In a typical application, two opposing magnetic actuators are used in combination with associated springs to control an armature connected to an engine valve stem. The upper actuator provides the upper force that attracts the armature and holds the valve in the closed position while the lower actuator provides the downward force that attracts the armature and holds the valve in the open position. The upper spring pushes the valve downward after the upper actuator is turned off while the lower spring pushes the valve upward after the lower actuator is turned off. The opening and closing or landing speed of the valve is a function of the spring force and the excitation current of the actuator.
Because of the magnetic property of the materials used for the armature and the core in these actuators, the magnetic flux generated by the current supplied to the actuator saturates the magnetic material after the current exceeds a certain level. As a result, the magnetic force of the actuator increases very little once the current reaches the saturation level. For example, in a typical material used for valve actuators in an internal combustion engine, once saturation of the core and armature is reached, an increase of 300% in the excitation current may result in only a 14% increase in the magnetic force.
For many applications, it is desirable to provide fast, controlled valve actuation to improve engine performance without a significant increase in the power consumption of the actuator, which would adversely affect fuel economy. As such, it is desirable to provide actuators having high force density (force/volume), which leads to faster valve actuation and lower power consumption.
Permanent magnets have been used in combination with electromagnets to provide a holding force and/or to increase the magnetic force of the actuator without significant additional power consumption. For example, U.S. Pat. Nos. 4,779,582 and 4,829,947 disclose actuators that have permanent magnets. However, the disclosed constructions having permanent magnets positioned laterally to the outside of the armature of these actuators makes it difficult to control the magnetic flux because the permanent magnets impede the flux produced by the current of the electromagnet. As a result, it may be very difficult to control the armature and valve landing speed, which may result in undesirable noise and/or wear of the valve or valve seat. In addition, the flux through the permanent magnets of these arrangements varies over a wide range as the armature moves. This may lead to undesirable eddy current losses in the permanent magnets. Furthermore, because these actuators are designed to provide a holding force for the armature without any current supplied to the electromagnet, the permanent magnet flux results in a corresponding magnetic force after the current in the coil becomes zero such that the release of the armature from the core is delayed and the power consumption of the actuator is increased.
The present invention provides a valve actuator particularly suited for use in actuation of intake and/or exhaust valves of an internal combustion engine. In one embodiment, the actuator includes at least one electromagnet having a coil wound about a core, and an armature fixed to an armature shaft extending axially through the coil and the core, and axially movable relative thereto. The actuator includes at least one permanent magnet positioned between the coil and the armature. The permanent magnet(s) is/are preferably oriented so that magnetic flux of the permanent magnet(s) travels in a direction opposite to magnetic flux generated by the coil through the core to reduce saturation of the core, but in the same direction as the magnetic flux generated by the coil through the armature, to increase an attractive force between the armature and the electromagnet. The actuator may also include a valve that functions as an intake or exhaust valve for an internal combustion engine. The valve includes a valve stem operatively associated with the armature shaft for axial movement therewith. At least one spring is associated with the valve stem or armature shaft to overcome the magnetic attractive force of the permanent magnet and move the armature away from the electromagnet when the electromagnet coil is de-energized. In a typical application, upper and lower electromagnets and springs are provided to open and close the intake/exhaust valve in response to energization of the corresponding upper (close) and lower (open) electromagnet coils.
Alternative embodiments of the present invention include an E-core actuator having a generally oval coil and two rectangular permanent magnets positioned between the coil and the armature, and a pod-core actuator having a generally circular coil and a single annular permanent magnet positioned between the coil and the armature.
The present invention provides a number of advantages. For example, actuators incorporating the present invention have the same flux controllability of conventional actuators because the permanent magnets do not block the flux produced by the current in the coil. As such, the present invention allows acceptable control of the armature speed. The construction of the present invention positions the permanent magnets so the majority of the associated flux travels through the core such that it does not vary significantly as the armature moves. Therefore, the eddy current losses in the permanent magnets are much lower than that of the previous actuators utilizing permanent magnets. Additionally, because most of the permanent magnet flux travels through the core and not to the armature, the magnetic force produced by the permanent magnet flux is very small. Therefore, the armature can be released with little delay and without higher power consumption compared to the conventional actuators.
Positioning of one or more permanent magnets according to the present invention allows the associated flux to travel against the flux produced by the coil in the core, while traveling with the flux produced by the coil in the air gap and through the armature. This reduces saturation of the core while increasing the attractive force of the armature such that the overall magnetic force produced by actuators according to the present invention is significantly higher for the same level of current relative to previous constructions. This increased force production capability can be used to decrease the transition time of the actuator through the use of stiffer springs to provide faster valve actuation, which improves the engine performance, and lower power consumption, which improves the engine fuel economy. Alternatively, the higher force density (force/volume) actuators according to the present invention allow a reduced size/weight actuator.
The above advantages and other advantages, objects, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings.